CARMA Memo #52

Aperture Efficiency, Pointing and Primary Beam Calibration

Melvyn Wright, David MacMahon, 
(Radio Astronomy Laboratory, U.C. Berkeley) 
Stuartt Corder (ALMA) 
and 
Goran Sandell (Universities Space Research Association)

ABSTRACT

In this memo we present an analysis of observations of compact sources in
an interferometer mosaic pattern to determine the aperture efficiency,
pointing, and primary beam width for the CARMA antennas. The data are
self-calibrated to determine the antenna voltage gains. Gaussian fits to
the antenna voltage patterns are used to analyze the CARMA beam patterns
at 100 & 230 GHz.  For multichannel observations the antenna bandpass
solutions are also obtained.

Using a hexagonal 7-pointing mosaic pattern works very well at 100
GHz. At 230 GHz good atmospheric phase stability is required to obtain
reliable results.  We measure antenna gains close to 65 and 145 Jy/K
corresponding to aperture efficiencies 50% and  65%  respectively for
10 and 6m antennas.  At 100 GHz the data are consistent with a common
FWHM for azimuth and elevation axes. The average FWHM for 10m antennas
is 1.20 +/- 0.02 arcmin. The average for 6m antennas is 1.98 +/- 0.03
arcmin.  These values are significantly larger than the the current
Gaussian primary beam model FWHM; 1.07 and 1.92 arcmin for 10m and 6m
antennas respectively.  At 230 GHz the FWHM on the elevation axis appears
to be smaller than on the azimuth for several of the antennas. This is
possibly caused by gravitational deformation in both 6m and 10m antennas.
The mean values are, azimuth: 0.55 +/- 0.02, elevation: 0.50 +/- 0.02
for 10m antennas, and azimuth: 0.86 +/- 0.02, elevation: 0.83 +/- 0.03
for 6m antennas.

In principle, we obtain an absolute gain calibration using planet
observations with a planet model. However, if the planet is resolved then
the derived gains depend on the detailed surface brightness distribution
in the planet model, and may not produce reliable fits for the aperture
efficiency or primary beamwidth.  For use as an absolute flux density
calibration, correction must be made for absorption lines in planetary
atmospheres.